Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). A MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MR images may be created by applying currents to the gradient and RF coils according to known algorithms called “pulse sequences.” The selection of a pulse sequence determines the relative appearance of different tissue types in the resultant images. Various properties of tissue may be used to create images with a desirable contrast between different tissues. T2* (T2 star) or susceptibility weighted contrast arises from local inhomogeneities of the magnetic field among tissues. T2* weighted contrast may be used for a number of applications including, but not limited to, enhanced detection of stroke and hemorrhage, tumors, micro-hemorrhages in trauma patients and occult vascular diseases (e.g., cavernomas, angiomas and telengiectasias), and for applications such as separating arteries and veins, imaging venous vascular networks and measuring iron buildup in neurodegenerative diseases.
Typically, conventional gradient echo (GRE) or echo planar imaging (EPI) pulse sequences are used to achieve T2* (or susceptibility) weighted contrast. GRE and EPI sequences use a gradient reversal to form an echo rather than an RF refocusing pulse. Accordingly, GRE and EPI sequences are sensitive to the magnetic susceptibility of different tissue types. The differences in magnetic susceptibility between tissue types causes magnetic field inhomogeneity and signal loss. GRE and EPI sequences used for T2* weighted imaging, however, may have a low signal-to-noise ratio (SNR) and low spatial resolution capability that can limit the applications for T2* weighted contrast imaging. It would be desirable to provide a method and apparatus for generating (e.g., acquisition and reconstruction) a T2* weighted MRI image that improves susceptibility sensitivity and SNR.